1. Field of the Invention
The present invention relates to a solar cell module comprising a plurality of photovoltaic elements (or solar cells) having an overvoltage preventive element and a sunlight power generation system using said solar cell module.
2. Related Background Art
A solar cell has such a property that because of its characteristics, the voltage is varied depending on a given load factor. For instance, the open-circuit voltage of a solar cell in a non-loaded state is always greater than the optimum operating voltage at which the output of the solar cell becomes maximum. In order for a solar cell to be efficiently used as a power generation device, the purpose can be achieved by actuating the solar cell at an optimum operating point (that is, by imparting an optimum load resistance to the solar cell). However, in the case where the load is lightened (that is, the load resistance value is enlarged), the voltage of the direct current circuit of the solar cell is increased, where when the solar cell is in a stopped state, the voltage is increased up to aforesaid open-circuit voltage. In this connection, for a power converter or the like to be connected to the direct current circuit of the solar cell, it is necessary to withstand against said voltage.
Incidentally, in the case of an ordinary amorphous silicon (a-Si) solar cell whose semiconductor layer comprises an a-Si material, it is known that the fill factor (F.F.) of the a-Si solar cell is inferior to that of a single crystal silicon solar cell whose semiconductor layer comprises a single crystal silicon material, and when the optimum operating voltage at which the output of the a-Si solar cell becomes maximum is the same as that at which the output of the single crystal silicon solar cell becomes maximum, the open-circuit voltage of the a-Si solar cell is greater than that of the single crystal silicon solar cell, and wherein when the open-circuit voltage is excessively large, overvoltage is generated.
In this connection, in the case of a sunlight power generation system in which such a-Si solar cell is used, an electric device such as a power converter or the like used therein is necessary to withstand against an increased voltage.
As a measure to comply with overvoltage which is generated when the open-circuit voltage is large as above described, an overvoltage preventive means is described in Japanese Patent No. 2580185 (issued Nov. 21, 1996)[hereinafter referred to as document 1].
Description will be made of the overvoltage preventive means described in document 1 with reference to FIG. 1. In FIG. 1, reference numeral 101 indicates a solar cell panel, reference numeral 102 a reverse current preventive diode, reference numeral 103 a voltage limit element, reference numeral 104 a constant-voltage D.C. power source, reference numeral 105 a compressor driving-motor, reference numeral 106 an inverter power module, and reference numeral 107 an inverter air-conditioner.
As shown in FIG. 1, to a serialized body (that is, a solar cell array) comprising the solar cell panel 101 and the reverse current preventive diode 102, the voltage limit element 103 is connected in parallel connection, and the paralleled circuit is connected to the compressor driving-motor 105 as a load. In the case where the quantity of solar radiation is large and the load is small, by making a dump power caused in the solar cell panel 101 to be consumed in the voltage limit element 103, occurrence of overvoltage is prevented.
Besides, another overvoltage preventive means is described in Japanese Patent Publication No. 89302/1995 (hereinafter referred to as document 2). Description will be made of the overvoltage preventive means described in document 2 with reference to FIG. 2. In FIG. 2, reference numeral 201 indicates a solar cell, reference numeral 202 an inverter, reference numeral 203 a control circuit, reference numeral 204 a short-circuiting switch, reference numeral 205 a voltage-dividing circuit, and reference numeral 206 an induction motor.
As shown in FIG. 2, a plurality of solar cells 201 are electrically serialized into a solar cell array, where the switch 204 for short-circuiting part of the solar cell array (the serialized solar cells) and the control circuit 203 for controlling the opening and closing of the switch 204 are provided. In the case where the generated voltage of the solar cell array becomes excessive, the control circuit 203 is actuated to close the switch 204 thereby to short-circuit part of the solar cell array, whereby overvoltage is prevented from occurring.
However, for these overvoltage preventive means, there are problems as will be described below.
In the case described in document 1, because the voltage limit element 103 is connected to the solar cell array (the serialized body) in parallel connection, the dump power caused in the solar cell array is entirely consumed in the voltage limit element. In this connection, there are such problems as will be described in the following. That is, the more the output of the solar cell array is increased, the more the power consumed in the voltage limit element is increased, where, for instance, when the output of the solar cell array becomes some hundreds wattage (W) or more, the power consumed in the voltage limit element is markedly increased and as a result, the voltage limit switch is extremely heated. Therefore, it is necessary to establish a large space for radiating heat from the voltage limit switch to cool the voltage limit switch.
In the case described in document 2, there are problems such that in order to short-circuit part of the solar cell array, extra short-circuiting wires which are extending from a prescribed position of the solar cell array, the specialized switch for the short-circuiting, the specialized control circuit for controlling the opening and closing of the short-circuiting switch, and the specialized power source for driving the control circuit are necessitated, and because of this, the system involved is unavoidably complicated.